A spark plug used for ignition in an internal combustion engine, such as gasoline engine, has a metal fitting for mounting the spark plug to a cylinder head of the engine. The metal fitting of the spark plug generally includes: a thread portion formed with an external thread for screwing into a threaded hole of the engine cylinder head; a tool engagement portion formed engageable with a tool e.g. a spark plug wrench; a seal portion formed between the thread portion and the tool engagement portion at a position continuous to the thread portion so as to ensure the gas-tightness of the inside of the engine; and a thin compression deformation portion formed between the seal portion and the tool engagement portion.
Conventionally, the metal fitting of the spark plug is completed through the processes of cold forging, cutting and thread forming. By the cold forging process, the metal fitting is provided in semi-finished form close to the finished product (see, for example, Japanese Laid-Open Patent Publication No. H07-016693).
As discussed in Japanese Laid-Open Patent Publication No. H07-016693, the cold forging process of the conventional metal fitting manufacturing method contains a plurality of steps. In the step of formation of the tool engagement portion of the finished metal fitting product, it is conceivable to form the tool engagement portion by “drawing” or “bulging”.
Herein, “drawing” and “bulging” operations for the formation of the tool engagement portion will be described below with reference to FIG. 11. FIG. 11 is a schematic view showing drawing and bulging operations in the cold forging process. FIGS. 11(A) and (B) shows one example of drawing operation; and FIGS. 11(C) and (D) shows one example of bulging operation. FIG. 11 refers to the case where the tool engagement portion is formed in a substantially regular hexagonal cross-sectional shape. When the tool engagement portion is formed by drawing a base material of substantially cylindrical column shape (including hollow cylindrical column shape), a diagonal dimension of the tool engagement portion becomes smaller than or equal to an outer diameter of the base material as shown in FIGS. 11(A) and (B). When the tool engagement portion is formed by bulging a base material of substantially cylindrical column shape, a diagonal dimension of the tool engagement portion becomes larger than or equal to an outer diameter of the base material as shown in FIGS. 11(C) and (D). When the tool engagement portion is formed in a non-regular polygonal cross-sectional shape by drawing, a maximum diagonal dimension (longest diagonal length) of the tool engagement portion becomes smaller than or equal to an outer diameter of the base material. When the tool engagement portion is formed in a non-regular polygonal cross-sectional shape by bulging, a minimum diagonal dimension (shortest diagonal line length) of the tool engagement portion becomes larger than or equal to an outer diameter of the base material.
To manufacture the metal fitting in which an outer diameter of the seal portion is smaller than the maximum diagonal dimension of the tool engagement portion, the tool engagement portion is conventionally formed by either of the following two processes: 1) forming the tool engagement portion by bulging; and 2) enlarging a part of the base material to an outer diameter larger than the maximum diagonal dimension of the tool engagement portion, thereby providing a portion to be formed into the seal portion (hereinafter referred to as “pre-seal portion”) integral with a portion to be formed into the tool engagement portion (hereinafter referred to as “pre-tool engagement portion”), and then, forming the tool engagement portion by drawing (see Japanese Laid-Open Patent Publication No. H07-016693).
The process 1) has the problem that the die for the bulging is high in cost and short in lifetime. The process 2) attains a reduction of die cost as compared to the process 1). In the process 2), however, the pre-seal portion and the pre-tool engagement portion are integrally formed by diameter enlargement such that the outer diameter of the pre-seal portion becomes larger than the outer diameter of the seal portion. The process 2) thus causes an increase of cutting amount during the formation of the seal portion in the cutting process, which leads to the problem of increase in chip treatment workload, deterioration in cutting edge lifetime, increase in material input etc. In other words, these conventional processes face the common problem of high manufacturing cost. This problem is common to various metal fittings with tool engagement portions, such as those for not only spark plugs but also sensors e.g. temperature sensors and other devices. Consequently, there has been a demand to develop a technique for reducing the manufacturing cost of metal fittings.